Image forming apparatus

ABSTRACT

An image forming apparatus includes an image bearing member, an image forming portion, a transfer member, a power source, an acquiring portion, and a controller capable of executing an operation in an adjustment mode. A plurality of test toner images transferred onto a recording material in the occurrence in the adjusting mode include a first test toner image transferred under application of a first test voltage and a second test toner image transferred under application of a second test voltage different from the first test voltage. In the operation in the adjustment mode, the controller adjusts the transfer voltage set for transfer on the basis of first information on variation in image density acquired in different regions of the first test toner image and second information on variation in image density acquired in different regions of the second test toner image.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image forming apparatus, such as aprinter, a copying machine, a facsimile machine, or a multi-functionmachine, of an electrophotographic type.

In an image forming apparatus, such as the printer, the copying machine,the facsimile machine, or the multi-function machine, of theelectrophotographic type, a toner image formed on an image bearingmember such as a photosensitive member or an intermediary transfermember is transferred onto a recording material. The transfer of thetoner image from an image bearing member to the recording material isperformed by applying a transfer voltage to a transfer member such as atransfer roller which contacts the image bearing member to form atransfer portion (transfer nip). The transfer voltage can be determinedbased on a transfer portion part voltage corresponding to the electricalresistance of the transfer portion detected during the pre-rotationprocess before image formation, and a recording material part voltagedepending on a kind of the recording material. By doing so, anappropriate transfer voltage can be set according to the environmentalfluctuations, the transfer member usage history, the recording materialkind, and the like.

However, there are various types and conditions of recording materialsused in the image formation, and therefore, the preset recordingmaterial part voltage may be higher or lower than the appropriatetransfer voltage. Under the circumstances, an image forming apparatusoperable in an adjustment mode in which set voltage (value) of asecondary transfer voltage is adjusted depending on the recordingmaterial actually used in the image formation is proposed, although inthe case of an intermediary transfer type including an intermediarytransfer belt as an image bearing member (Japanese Laid-open PatentApplication No. 2013-37185). In the operation in the adjusting mode, aplurality of test toner images which are called patches are formed on asingle recording material while switching the secondary transfervoltage. The recording material on which the plurality of patches areformed is called an adjusting chart. Then, the recording material partvoltage is changed on the basis of an average image density acquiredfrom densities of the respective patches formed on the adjusting chart,whereby the secondary transfer voltage set during image formation isadjusted.

Incidentally, in the above-described conventional image formingapparatus, image defect such that the recording material is electricallydischarged in the neighborhood of the secondary transfer portion duringsecondary transfer and a charge polarity of toner is reversed at anassociated portion and the toner is not transferred onto the recordingmaterial and results in a white void in a dot shape (hereinafter, called“white void”) can occur.

The “white void” is liable to occur with an increasing secondarytransfer voltage, and is particularly liable to be visualized on ahalf-tone black image. In the case of the conventional image formingapparatus in which the set voltage of the secondary transfer voltage isadjusted on the basis of the above-described image density of thepatches, there was a liability that the set voltage of the secondarytransfer voltage is adjusted to a level (absolute value) at which thewhite void is caused to occur. In view of this, an image formingapparatus in which an upper limit is provided to the recording materialpart voltage and in which the secondary transfer voltage set during theimage formation within a range in which the occurrence of the “whitevoid” can be suppressed is adjusted, is proposed (Japanese Laid-OpenPatent Application No. 2020-144289).

However, conventionally, by providing the upper limit to the recordingmaterial part voltage, the “white void” due to an excessively hightransfer voltage does not occur, but the transfer voltage was adjustedto a low level to the extent such that image density lowering is causedto occur, in some instances.

SUMMARY OF THE INVENTION

In view of the above-described problem, a principal object of thepresent invention is to provide an image forming apparatus capable ofadjusting a transfer voltage to a transfer voltage at which suppressionof occurrence of a “white void” and suppression of occurrence of an“image density lowering” are compatibly realized in the case of aconstitution in which an occurrence in an adjusting mode in which thetransfer voltage set for image formation on the basis of an adjustingchart is adjusted.

According to an aspect of the present invention, there is provided animage forming apparatus comprising: an image bearing member configuredto bear a toner image; an image forming portion configured to form thetoner image on the image bearing member; a transfer member configured totransfer the toner image from the image bearing member onto a recordingmaterial; a power source configured to apply a transfer voltage, to thetransfer member, for transferring the toner image from the image bearingmember onto the recording material; an acquiring portion configured toacquire information on an image density of an image transferred on therecording material; and a controller capable of executing an operationin an adjustment mode in which a test chart is outputted by transferringa plurality of test toner images onto the recording material underapplication of a plurality of different test voltages from the powersource and then on the basis of densities of the test toner imagestransferred on the test chart, the transfer voltage set for transfer ofthe toner image from the image bearing member onto the recordingmaterial is adjusted, wherein the test toner images include a first testtoner image transferred onto the recording material under application ofa first test voltage and a second test toner image transferred onto therecording material under application of a second test voltage differentfrom the first test voltage, and wherein in the operation in theadjustment mode, the controller adjusts the transfer voltage set for thetransfer on the basis of first information on variation in image densityacquired in different regions of the first test toner image and secondinformation on variation in image density acquired in different regionsof the second test toner image.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an image forming apparatus of anembodiment.

FIG. 2 is a block diagram showing a schematic structure of a controlsystem of the image forming apparatus.

FIG. 3 is a flowchart showing control of a secondary transfer voltage.

FIG. 4 is a graph showing a voltage-current characteristic acquired inthe control of the secondary transfer voltage.

FIG. 5 is a table showing table data of a recording material partvoltage.

FIG. 6 is a schematic illustration of large chart data.

Parts (a) and (b) of FIG. 7 are schematic illustrations of small chartdata, in which part (a) shows the small chart data on a first side, andpart (b) shows the small chart data on a second side.

FIG. 8 is a flowchart showing an operation in an adjustment mode in afirst embodiment.

FIG. 9 is a schematic illustration showing a setting screen of asecondary transfer voltage.

FIG. 10 is a graph showing a relationship between an adjusting value ofthe secondary transfer voltage and an average of brightness of a patch.

FIG. 11 is a schematic illustration showing a method of acquiringbrightness data for calculating a brightness dispersion value.

FIG. 12 is a graph for illustrating a relationship between “white void”and the brightness dispersion value.

FIG. 13 includes graphs for illustrating a method of discriminatingoccurrence or non-occurrence of the “white void”.

FIG. 14 is a graph for illustrating a relationship between a recordingmaterial part voltage and liability to occurrence of the “white volt”.

FIG. 15 is a table showing table data of an upper limit of the recordingmaterial part voltage.

FIG. 16 includes graphs for illustrating a changing method of theadjusting value when the “white void” occurred.

FIG. 17 is a flowchart showing an operation in an adjusting mode in asecond embodiment.

FIG. 18 is a schematic illustration showing a setting screen of asecondary transfer voltage in the second embodiment.

FIG. 19 is a schematic illustration showing a setting screen of apriority image.

DESCRIPTION OF EMBODIMENTS First Embodiment

<Image Forming Apparatus>

In the following, an image forming apparatus according to a firstembodiment will be described with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus2 of this embodiment. The image forming apparatus 2 of this embodimentis a tandem type full-color printer capable of forming a full-colorimage by using an electrophotographic type process and employing anintermediary transfer type system. However, the image forming apparatus2 of the present invention is not limited to a tandem type image formingapparatus, and may be an image forming apparatus of another type. Inaddition, the image forming apparatus 2 is not limited to an imageforming apparatus capable of forming the full-color image, and may be animage forming apparatus capable of forming only a monochromatic(white/black or single color) image. Further, the image formingapparatus 2 may also be one of various-purpose image forming apparatusessuch as printers, various printing machines, copying machines, facsimilemachines and multi-function machines.

As shown in FIG. 1 , the image forming apparatus 2 includes a feedingportion 4, an image forming portion 5, a controller 30, and an operatingportion 70. In FIG. 1 , the single feeding portion 4 is provided, but aplurality of feeding portions 4 may also be provided. Inside theapparatus, a temperature sensor 71 capable of detecting the temperatureinside the apparatus and a humidity sensor 72 capable of detecting thehumidity inside the apparatus are provided (see FIG. 2 ). The imageforming apparatus 2 can form a four-color-based, full-color image on arecording material S, in accordance with image information (imagesignals) supplied from an image reading portion 80 as a reading meansfor reading an image on the sheet or from an external device 200 (seeFIG. 2 ). As the external device 200, it is possible to cite a hostdevice, such as a personal computer, or a digital camera or asmartphone. Here, the recording material S is a material on which atoner image is formed, and specific examples thereof include plainpaper, synthetic resin sheets which are substitutes for plain paper,thick paper, and overhead projector sheets.

The image forming portion 5 can form the image on the recording materialS fed from the feeding portion 4 and moved through an inside of afeeding path J, on the basis of the image information. The image formingportion 5 includes image forming units 50 y, 50 m, 50 c, 50 k, tonerbottles 41 y, 41 m, 41 c, 41 k, exposure devices 42 y, 42 m, 42 c, 42 k,an intermediary transfer unit 44, a secondary transfer device 45, and afixing portion 46. The image forming units 50 y, 50 m, 50 c, and 50 kform yellow (y), magenta (m), cyan (c), and black (k) images,respectively. Elements having the same or corresponding functions orstructures provided for these four image forming units 50 y, 50 m, 50 c,and 50 k are collectively described in some instances by omittingsuffixes y, m, c and k of reference numerals or symbols representing theelements for associated colors. Here, the image forming apparatus 2 canalso form a single-color or multi-color image by using an image formingunit 50 for a desired single color or some of the four colors, such as amonochromatic black image.

The image forming unit 50 includes the following means. First, aphotosensitive drum 51 which is a drum-type (cylindrical) photosensitivemember (electrophotographic photosensitive member) is provided. Inaddition, a charging roller 52 which is a roller-type charging member isprovided. In addition, a developing device 20 is provided. In addition,a pre-exposure device 54 is provided. In addition, a cleaning blade 55is provided. The image forming unit 50 forms a toner image on theintermediary transfer belt 44 b which will be described hereinafter. Theimage forming unit 50 is integrally assembled into a unit as a processcartridge and can be mounted to and dismounted from an apparatus mainassembly 10.

The photosensitive drum 51 is movable (rotatable) while carrying anelectrostatic image (electrostatic latent image) or a toner image. Inthis embodiment, the photosensitive drum 51 is a negatively chargeableorganic photosensitive member (organic photoconductor: OPC) having anouter diameter of 30 mm. The photosensitive drum 51 has an aluminumcylinder as a base material and a surface layer formed on the surface ofthe base material. In this embodiment, the surface layer comprises threelayers of an undercoat layer, a photocharge generation layer, and acharge transportation layer, which are applied and laminated on thesubstrate in the order named. When an image forming operation isstarted, the photosensitive drum 51 is driven to rotate in a directionindicated by an arrow (counterclockwise) in the figure at apredetermined process speed (circumferential speed) by a motor (notshown).

The surface of the rotating photosensitive drum 51 is uniformly chargedby the charging roller 52. In this embodiment, the charging roller 52 isa rubber roller which contacts the surface of the photosensitive drum 51and which is rotated by the rotation of the photosensitive drum 51. Thecharging roller 52 is connected with a charging bias (voltage) powersource 73 (see FIG. 2 ). The charging bias (voltage) power source 73applies a charging bias (charging voltage) to the charging roller 52during the charging process.

The surface of the charged photosensitive drum 51 is scanned and exposedby the exposure device 42 in accordance with the image information, sothat an electrostatic image is formed on the photosensitive drum 51. Theexposure device 42 includes a laser scanner in this embodiment. Theexposure device 42 emits laser beam in accordance with the separatedcolor image information outputted from the controller 30, and scans andexposes the surface (outer peripheral surface) of the photosensitivedrum 51.

The electrostatic image formed on the photosensitive drum 51 isdeveloped (visualized) by supplying the developer toner thereto by thedeveloping device 20, so that a toner image is formed on thephotosensitive drum 51. In this embodiment, the developing device 20contains a two-component developer (also simply referred to as“developer”) comprising non-magnetic toner particles (toner) andmagnetic carrier particles (carrier). The toner is supplied from thetoner bottle 41 to the developing device 20. The developing device 20includes a developing sleeve 24. The developing sleeve 24 is made of anonmagnetic material such as aluminum or nonmagnetic stainless steel(aluminum in this embodiment). Inside the developing sleeve 24, a magnetroller, which is a roller-shaped magnet, is fixed and arranged so as notto rotate relative to the main body (developing container) of thedeveloping device 20. The developing sleeve 24 carries a developer andconveys it to a developing zone facing the photosensitive drum 51. Adeveloping bias power source 74 (see FIG. 2 ) is connected to thedeveloping sleeve 24. The developing bias power source 74 applies adeveloping bias (developing voltage) to the developing sleeve 24 duringthe developing process operation. In this embodiment, the normalcharging polarity of the toner, which is the charging polarity of thetoner during development, is negative.

An intermediary transfer unit 44 is arranged so as to face the fourphotosensitive drums 51 y, 51 m, 51 c, 51 k. The intermediary transferunit 44 includes an intermediary transfer belt 44 b, constituted by anendless belt, as a second image bearing member. The intermediarytransfer belt 44 b is wound around a plurality of rollers such as adriving roller 44 a, a driven roller 44 d, primary transfer rollers 47y, 47 m, 47 c, 47 k, and an inner secondary transfer roller 45 a. Theintermediary transfer belt 44 b is movable (rotatable) carrying thetoner image. The driving roller 44 a is rotationally driven by a motor(not shown) as driving means, and rotates (circulates) the intermediarytransfer belt 44 b. The driven roller 44 d is a tension roller whichcontrols the tension of the intermediary transfer belt 44 b to beconstant. The driven roller 44 d is subjected to a force which pushesthe intermediary transfer belt 44 b toward the outer peripheral surfaceby the urging force of a spring (not shown), and by this force, atension of about 2 to 5 kg is applied in a process advance direction ofthe intermediary transfer belt 44 b. The inner secondary transfer roller45 a constitutes the secondary transfer device 45 as will be describedhereinafter. The driving force is transmitted to the intermediarytransfer belt 44 b by the driving roller 44 a, and the intermediarytransfer belt 44 b is rotationally driven in the arrow direction(clockwise) in the drawing at a predetermined peripheral speedcorresponding to the peripheral speed of the photosensitive drum 51. Inaddition, the intermediary transfer unit 44 is provided with a beltcleaning device 60 as intermediary transfer member cleaning means.

The primary transfer rollers 47 y, 47 m, 47 c, 47 k are arranged to facethe photosensitive drums 51 y, 51 m, 51 c, 51 k, respectively. Theprimary transfer roller 47 holds the intermediary transfer belt 44 bbetween the photosensitive drum 51 and the primary transfer roller 47.By this, the intermediary transfer belt 44 b contacts the photosensitivedrum 51 to form a primary transfer portion (primary transfer nipportion) 48 with the photosensitive drum 51.

The toner image formed on the photosensitive drum 51 is primarilytransferred onto the intermediary transfer belt 44 b by the action ofthe primary transfer roller 47 in the primary transfer portion 48. Thatis, in this embodiment, by applying a positive primary transfer voltageto the primary transfer roller 47, a negative toner image on thephotosensitive drum 51 is primarily transferred onto the intermediarytransfer belt 44 b. For example, when forming a full-color image, theyellow, magenta, cyan, and black toner images formed on thephotosensitive drums 51 y, 51 m, 51 c, and 51 k are transferred so as tobe sequentially superimposed on the intermediary transfer belt 44 b. Aprimary transfer power source 75 (see FIG. 2 ) is connected to theprimary transfer roller 47. The primary transfer power supply 75 appliesa DC voltage having a polarity opposite to the normal charging polarityof the toner (positive polarity in this embodiment) as a primarytransfer bias (primary transfer voltage) to the primary transfer roller47 during the primary transfer process operation. The primary transferpower supply 75 is connected to a voltage detection sensor 75 a whichdetects the output voltage and a current detection sensor 75 b whichdetects the output current (see FIG. 2 ). In this embodiment, theprimary transfer power sources 75 y, 75 m, 75 c, and 75 k are providedfor the primary transfer rollers 47 y, 47 m, 47 c, and 47 k,respectively, and the primary transfer voltages applied to the primarytransfer rollers 47 y, 47 m, 47 c and 47 k can be individuallycontrolled.

The primary transfer roller 47 has an elastic layer of ion conductivefoam rubber (NBR rubber) and a cored bar. The outer diameter of theprimary transfer roller 47 is, for example, 15 to 20 mm. In addition, asthe primary transfer roller 47, a roller having an electric resistancevalue of 1×10⁵ to 1×10⁸Ω (N/N (23° C., 50% RH) condition, 2 kV applied)can be preferably used.

The intermediary transfer belt 44 b is an endless belt having atwo-layer structure including a base layer and a surface layer in theorder named from the inner peripheral surface side. As the resinmaterial constituting the base layer, a resin such as polyimide orpolycarbonate, or a material containing an appropriate amount of carbonblack as an antistatic agent in various rubbers can be suitably used.The thickness of the base layer is, for example, 0.05 to 0.15 [mm]. As amaterial constituting the surface layer, a resin such as a fluororesincan be suitably used. The surface layer has small adhesive force of thetoner to the surface of the intermediary transfer belt 44 b and makes iteasier to transfer the toner onto the recording material S at asecondary transfer portion 45 n. The thickness of the surface layer is,for example, 0.0002 to 0.020 [mm]. In this embodiment, for the surfacelayer, one kind of resin material such as polyurethane, polyester, epoxyresin, or two or more kinds of elastic materials such as elasticmaterial rubber, elastomer, butyl rubber, for example, are used as abase material. And, as a material for reducing the surface energy andimproving the lubricity of this base material, powder or particles suchas fluororesin, for example, with one kind or two kinds or differentparticle diameters are dispersed, so that a surface layer is formed. Inthis embodiment, the intermediary transfer belt 44 b has a volumeresistivity of 5×10⁸ to 1×10¹⁴[Ω·cm](23° C., 50% RH) and a staticfriction coefficient of the intermediary transfer belt 44 b is 0.15 to0.6 (23° C., 50% RH, type 94i manufactured by HEIDON). In thisembodiment, the two-layer structure was employed, but a single-layerstructure of a material corresponding to the material of the base layermay also be employed.

On the outer peripheral surface side of the intermediary transfer belt44 b, an outer secondary transfer roller 45 b which constitutes thesecondary transfer device 45 in cooperation with the inner secondarytransfer roller 45 a is disposed. The outer secondary transfer roller 45b contacts the intermediary transfer belt 44 b contacting the innersecondary transfer roller 45 a and forms the secondary transfer portion(secondary transfer nip portion) 45 n between the intermediary transferbelt 44 b. The toner image formed on the intermediary transfer belt 44 bis secondarily transferred onto the recording material S by the actionof the secondary transfer device 45 in the secondary transfer portion 45n. In this embodiment, a positive secondary transfer voltage is appliedto the outer secondary transfer roller 45 b so that the negative tonerimage on the intermediary transfer belt 44 b is secondarily transferredonto the recording material S which is nipped and fed between theintermediary transfer belt 44 b and the outer secondary transfer roller45 b. The recording material S is fed from the feeding portion 4 inparallel with the above-described toner image forming operation, and thetoner image on the intermediary transfer belt 44 b is fed by aregistration roller pair 11 provided in the feeding path J at the timingadjusted. The sheet is then fed to the secondary transfer portion N.

As described above, the secondary transfer device 45 is constituted byincluding an inner secondary transfer roller 45 a and an outer secondarytransfer roller 45 b as a transfer member. The inner secondary transferroller 45 a is disposed opposite to the outer secondary transfer roller45 b with the intermediary transfer belt 44 b interposed therebetween.To the outer secondary transfer roller 45 b, a secondary transfer powersupply 76 (see FIG. 2 ) is connected. During the secondary transferprocess, the secondary transfer power source 76 applies a DC voltagehaving a polarity opposite to the normal charging polarity of the toner(positive in this embodiment) to the outer secondary transfer roller 45b as secondary transfer bias (secondary transfer voltage). The secondarytransfer power source 76 is connected to a voltage detection sensor 76 afor detecting the output voltage and a current detection sensor 76 b fordetecting the output current (see FIG. 2 ). The core of the innersecondary transfer roller 45 a is connected to the ground potential.And, when the recording material S is supplied to the secondary transferportion 45 n, a secondary transfer voltage with constant-voltage-controlhaving a polarity opposite to the normal charging polarity of the toneris applied to the outer secondary transfer roller 45 b. In thisembodiment, a secondary transfer voltage of 1 to 6.5 kV is applied, acurrent of about 50 to 100 μA, for example is applied, and the tonerimage on the intermediary transfer belt 44 b is secondarily transferredonto the recording material S. Here, in this embodiment, the innersecondary transfer roller 45 a is connected to the ground potential, anda voltage is applied from the secondary transfer power source 76 to theouter secondary transfer roller 45 b. On the other hand, a voltage fromthe secondary transfer power source 76 is applied to the inner secondarytransfer roller 45 a, and the outer secondary transfer roller 45 b mayalso be connected to the ground potential. In such a case, a DC voltagehaving the same polarity as the normal charging polarity of the toner isapplied to the inner secondary transfer roller 45 a.

In this embodiment, the outer secondary transfer roller 45 b has anelastic layer of ion conductive foam rubber (NBR rubber) and a coremetal. The outer diameter of the outer secondary transfer roller 45 bis, for example, 20 to 25 mm. In addition, as the outer secondarytransfer roller 45 b, a roller having an electric resistance value of1×10⁵ to 1×10⁸Ω (measured at N/N (23° C., 50% RH), 2 kV applied) can bepreferably used.

The recording material S onto which the toner image has been transferredis fed to a fixing portion 46. The fixing portion 46 includes a fixingroller 46 a and a pressure roller 46 b. The fixing roller 46 a includestherein a heater. The recording material S carrying the unfixed tonerimage is heated and pressed by being sandwiched and fed between thefixing roller 46 a and the pressing roller 46 b. By this, the tonerimage is fixed (melted and fixed) on the recording material S. Here, thetemperature of the fixing roller 46 a (fixing temperature) is detectedby a fixing temperature sensor 77 (see FIG. 2 ).

In the case where image formation is carried out by one-side printing,the recording material S on which the toner image is fixed is fedthrough a discharge opening path 6 and is discharged through a dischargeopening, and then is stacked on a discharge tray 8 provided outside theapparatus main assembly 10. On the other hand, in the case where theformation of the image on the recording material S is carried out bydouble (both)-side printing, the recording material S on which the tonerimage is fixed on a first side is turned upside down and is suppliedagain to the secondary transfer portion 45 n. Then, the recordingmaterial S passes through a reverse feeding path 7 and is supplied tothe secondary transfer portion 45 n again, so that the toner image istransferred onto a second side and is fixed on the recording material S.Thereafter, the recording material S is fed along the discharge path 6and is stacked on the discharge tray 8 provided outside the apparatusmain assembly 10. As described above, the image forming apparatus 2 ofthis embodiment is capable of executing automatic double-side printingwhich forms images on both sides of a single recording material S.

The surface of the photosensitive drum 51 after the primary transfer iselectrically discharged by the pre-exposure device 54. In addition, thetoner remaining on the photosensitive drum 51 without being transferredonto the intermediary transfer belt 44 b during the primary transferprocess (primary untransferred residual toner) is removed from thesurface of the photosensitive drum 51 by the cleaning blade 55 and iscollected in a collection container (not shown). The cleaning blade 55is a plate-like member which is in contact with the photosensitive drum51 with a predetermined pressing force. The cleaning blade 55 is incontact with the surface of the photosensitive drum 51 in a counterdirection in which the outer end portion of the free end portion facesthe upstream side in the rotational direction of the photosensitive drum51. In addition, toner remaining on the intermediary transfer belt 44 bwithout being transferred onto the recording material S during thesecondary transfer process (secondary untransferred residual toner) oradhering matter such as paper dust is removed and collected from thesurface of the intermediary transfer belt 44 b by the belt cleaningdevice 60.

At an upper portion of the apparatus main assembly 10, an automaticoriginal feeding device 81 and an image reading portion 80 are provided.The automatic original feeding device 81 automatically feeds, toward theimage reading portion 80, a sheet (for example, an adjusting chartdescribed later) such as an original or the recording material S onwhich the image is formed. The image reading portion 80 as an acquiringportion reads the image on the sheet fed by the automatic originalfeeding device 81. The image reading portion 80 illuminates the sheetplaced on a plating glass 82 with light from a light source (not shown)and is constituted so as to read the image on the sheet, in terms of adot density determined in advance, by an image reading element (notshown). That is, the image reading portion 80 optically reads the imageon the sheet and converts the read image into an electric signal.

<Controller>

As shown in FIG. 1 , the image forming apparatus 1 of this embodimentincludes the controller 30, and operations of respective portions arecontrolled by the controller 30. The controller 30 will be describedusing FIG. 2 while making reference to FIG. 1 . The controller 30 isconstituted by a computer, and includes, for example, a CPU (CentralProcessing Unit) 31, a ROM (Read Only Memory) 32, a RAM (Random AccessMemory) 33, and an input/output circuit (I/F) 34 forinputting/outputting signals to and from the outside. The CPU 31 is amicroprocessor which controls the entire image forming apparatus 2 andis a main part of the system controller. The CPU 31 is connected to thefeeding portion 4, the image forming portion 5, and the operatingportion 70 and the like via the input/output circuit (I/F) 34, andexchanges signals with these portions, and controls the operation ofeach of these portions. The ROM 32 stores an image formation controlsequence (program) for forming an image on the recording material S. Thecontroller 30 is connected to a charging bias power source 73, adeveloping bias power source 74, a primary transfer power source 75, anda secondary transfer power source 76, which are controlled by signalsfrom the controller 30, respectively. In addition, the controller 30 isconnected to a temperature sensor 71, a humidity sensor 72, a voltagedetection sensor 75 a and a current detection sensor 75 b of the primarytransfer power supply 75, a voltage detection sensor 76 a and a currentdetection sensor 76 b of the secondary transfer power supply 76, and afixing temperature sensor 77.

The operating portion 70 as an input portion includes an unshownoperation button, and a display portion 70 a including a liquid crystalpanel. In the case of this embodiment, the display portion 70 a isconstituted as a touch panel, and also has a function as the inputportion. The operators such as a user or a service person can execute ajob (a series of operations to form and output an image or images on oneor more recording materials S in response to one start instruction) andcan input various pieces of information by operating the operationportion 70. The controller 30 receives the signal from the operatingportion 70 and operates various devices of the image forming apparatus2. The image forming apparatus 2 can also execute the job on the basisof an image forming signal (image data, control command) supplied fromthe external device 200 such as the personal computer.

The controller 30 includes an image formation pre-preparation processportion 31 a, an ATVC process portion 31 b, an image formation processportion 31 c, and an adjustment process portion 31 d. In addition, thecontroller 30 includes a primary transfer voltage storage/operationportion 31 e and a secondary transfer voltage storage/operation portion31 f Here, each of these process portions and storage/operation portionsmay be provided as a portion or portions of the CPU 31 or the RAM 33.For example, the controller 30 (specifically the image formation processportion 31 c) can execute a print job as described above. In addition,the controller 30 (specifically the ATVC process portion 31 b) canexecute ATVC (setting mode) for the primary transfer portion and thesecondary transfer portion. The ATVC will be described hereinafter. Inaddition, the controller 30 (specifically the adjustment process portion31 d) can execute an operation in an adjustment mode for adjusting a setvalue of the secondary transfer voltage. The operation in the adjustmentmode will be described hereinafter.

Here, the image forming apparatus 2 executes the job (image outputoperation, print job) which is series of operations to form and outputan image or images on a single or a plurality of recording materials Sstarted by one start instruction. The job includes an image formingstep, a pre-rotation step, a sheet (paper) interval step in the casewhere the images are formed on the plurality of recording materials S,and a post-rotation step in general. The image forming step is performedin a period in which formation of an electrostatic image for the imageactually formed and outputted on the recording material S, formation ofthe toner image, primary transfer of the toner image and secondarytransfer of the toner image are carried out, in general, and duringimage formation (image forming period) refers to this period.Specifically, timing during the image formation is different amongpositions where the respective steps of the formation of theelectrostatic image, the toner image formation, the primary transfer ofthe toner image and the secondary transfer of the toner image areperformed. The pre-rotation step is performed in a period in which apreparatory operation, before the image forming step, from an input ofthe start instruction until the image is started to be actually formed,is performed. The sheet interval step is performed in a periodcorresponding to an interval between a recording material S and asubsequent recording material S when the images are continuously formedon a plurality of recording materials S (continuous image formation).The post-rotation step is performed in a period in which apost-operation (preparatory operation) after the image forming step isperformed. During non-image formation (non-image formation period) is aperiod other than the period of the image formation (during imageformation) and includes the periods of the pre-rotation step, the sheetinterval step, and the post-rotation step, and further includes a periodof a pre-multi-rotation step, which is a preparatory operation duringturning-on of a main switch (power source) of the image formingapparatus 1 or during restoration from a sleep state.

<Control of Secondary Transfer Voltage>

Next, control of the secondary transfer voltage will be described. FIG.3 is a flow chart showing an outline of a procedure relating to thecontrol of the secondary transfer voltage in this embodiment. Generally,the control of the secondary transfer voltage includes constant-voltagecontrol and constant-current control, and in this embodiment, theconstant-voltage control is used.

First, the controller 30 (image formation pre-preparation processportion 31 a) causes the image forming portion to start an operation ofa job when it acquires information on the job from the operation portion70 or the external device 200 (S101). In the information on this job,image information designated by the operator and information on therecording material S are included. The information on the recordingmaterial S includes information on a size of the recording material Sand information on a kind (category of paper kind) of the recordingmaterial S such as “thin paper, plain paper, thick paper, . . . ”.Incidentally, the kind of the recording material S includes featuresbased on general characteristics such as plain paper, thick paper, thinpaper, glossy paper, coated paper, and any distinguishable informationon the recording material S, such as brand, product number, basisweight, thickness. The controller 30 writes this job information in theRAM 33 (S102).

Next, the controller 30 (image formation pre-preparation process portion31 a) acquires environment information detected by the temperaturesensor 71 and the humidity sensor 72 (S103). In the ROM 32, informationshowing correction between the environment information and a targetcurrent Itarget for transferring the toner image from the intermediarytransfer belt 44 b onto the recording material S is stored. Thecontroller 30 (secondary transfer voltage storage/operation portion 31f) acquires the target current Itarget corresponding to the environmentfrom the information showing the correlation between the environmentinformation and the target current Itarget, on the basis of theenvironment information read in S103. Then, the controller 30 writesthis target current Itarget in the RAM 33 (or the secondary transfervoltage storage/operation portion 31 f) (S104). Incidentally, why thetarget current Itarget is changed depending on the environmentinformation is that the toner charge amount varies depending on theenvironment. The target current Itarget in this embodiment has beenacquired for every environment in advance by a study on a secondarytransfer current value at which a toner image (a secondary-color wholesuppress image in this embodiment) with a maximum toner applicationamount can be transferred onto the recording material S by the imageforming apparatus.

Next, the controller 30 (ATVC process portion 31 b) acquires informationon an electric resistance of the secondary transfer portion 45 n by ATVC(Active Transfer Voltage Control) before the toner image on theintermediary transfer belt 44 b and the recording material S onto whichthe toner image is transferred reach the secondary transfer portion 45 n(S105). That is, in a state in which the outer secondary transfer roller45 b and the intermediary transfer belt 44 b are contacted to eachother, predetermined voltages of a plurality of levels are applied(supplied) from the secondary transfer power source 76 to the outersecondary transfer roller 45 b. Then, current values when thepredetermined voltages are applied are detected by the current detectingsensor 76 b, so that a relationship between the voltage and the current(voltage-current characteristic) as shown in FIG. 4 is acquired. Thecontroller 30 writes information on this relationship between thevoltage and the current in the RAM 33. This relationship between thevoltage and the current changes depending on the electric resistance ofthe secondary transfer portion 45 n. In the constitution of thisembodiment, the relationship between the voltage and the current is notsuch that the current changes linearly relative to the voltage (i.e., islinearly proportional to the voltage), but is such that the currentchanges so as to be represented by a polynomial expression consisting oftwo or more terms of the voltage. For that reason, in this embodiment,in order that the relationship between the voltage and the current canbe represented by the polynomial expression, the number of predeterminedvoltages or currents supplied when the information on the electricresistance of the secondary transfer portion 45 n is acquired is threeor more (levels).

Then, the controller 30 (secondary transfer voltage storage/operationportion 31 f) acquires a voltage value to be applied from the secondarytransfer power source 76 to the outer secondary transfer roller 45 b(S106). That is, on the basis of the target current Itarget written inthe RAM 33 in S104 and the relationship between the voltage and thecurrent acquired in S105, the controller 30 acquires a voltage value Vbnecessary to cause the target current Itarget to flow in a state inwhich the recording material S is absent in the secondary transferportion 45 n. This voltage value Vb corresponds to a secondary transferportion part voltage (transfer voltage corresponding to the electricresistance of the secondary transfer portion 45 n). Further, in the ROM32, information for acquiring a recording material part voltage(transfer voltage corresponding to the electric resistance of therecording material S) Vp as shown in FIG. 5 is stored. This informationis set as table data between ambient water content and the recordingmaterial part voltage Vp for each of sections (corresponding to paperkind categories) of basis weights of recording materials S. The tabledata for acquiring the recording material part voltage Vp as shown inFIG. 5 is acquired in advance by an experiment or the like.Incidentally, the controller 30 (image formation pre-preparation processportion 31 a) is capable of acquiring water content in an externalenvironment (which may include the inside of the apparatus mainassembly) on the basis of environment information (temperature,humidity) detected by the temperature sensor 71 and the humidity sensor72.

On the basis of the information on the job acquired in S101 and theenvironment information acquired in S103, the controller 30 acquires therecording material part voltage Vp from the above-described table data.Further, in the case where the adjusting value is set by the operationin the adjustment mode, described later, for setting the set voltage ofthe secondary transfer voltage, an adjusting value ΔV is determineddepending on the adjusting value. As described later, this adjustingvalue ΔV is stored in the RAM 33 (or the secondary transfer voltagestorage/operation portion 31 f) in the case where the adjusting value isset by the operation in the adjustment mode. The controller 30 acquiresVb+Vp+ΔV which is the sum of the above-described voltage values Vb, Vpand ΔV, as a secondary transfer voltage Vtr applied from the secondarytransfer power source 76 to the outer secondary transfer roller 45 bwhen the recording material S passes through the secondary transferportion N2. Then, the controller 30 writes this Vtr (=Vb+Vp+ΔV) in theRAM 33 (or the secondary transfer voltage storage/operation portion 31f).

Here, the recording material part voltage Vp also changes depending on asurface property of the recording material S other than the information(thickness, basis weight or the like) relating to the resistance of therecording material S in some instances. For that reason, the table datamay also be set so that the recording material part voltage Vp changesalso depending on the information relating to the surface property ofthe recording material S. Further, in this embodiment, the informationrelating to the resistance of the recording material S (and in addition,the information relating to the surface property of the recordingmaterial S) are included in the job information acquired in S101.However, a measuring means for detecting the thickness of the recordingmaterial S and the surface property of the recording material S isprovided in the image forming apparatus 2, and the recording materialpart voltage Vp may also be acquired on the basis of informationacquired by this measuring means.

Next, the controller 30 (the image formation process portion 31 c)causes the image forming portion to form the image and to send therecording material S to the secondary transfer portion 45 n and causesthe secondary transfer device to perform the secondary transfer byapplying the secondary transfer voltage Vtr determined as describedabove (S107). Thereafter, the controller 30 (the image formation processportion 31 c) repeats the processing of S107 until all the images in thejob are transferred and completely outputted on the recording material S(S108).

Incidentally, also as regards the primary transfer portion 48, the ATVCsimilar to the above-described ATVC is carried out in a period from astart of the job until the toner image is fed to the primary transferportion 48, but detailed description thereof will be omitted in thisembodiment.

<Outline of Adjustment Mode>

Next, an operation in a simple adjustment mode (hereinafter simplyreferred to as an “adjustment mode) for setting the set voltage of thesecondary transfer voltage will be described. Depending on the type andcondition of the recording material S used in image formation, the kindwater (moisture) content and electrical resistance value of therecording material S may differ greatly from the standard recordingmaterial S. In this case, optimal transfer may not be performed with theset voltage of the secondary transfer voltage using the defaultrecording material part voltage Vp set in advance as described above.

First, the secondary transfer voltage needs to be a voltage necessaryfor transferring the toner from the intermediary transfer belt 44 b tothe recording material S. In addition, the secondary transfer voltagemust be suppressed to a voltage level with which the abnormal dischargedoes not occur. However, depending on the type and state of therecording material S actually used for image formation, the electricalresistance may be higher than the value assumed as a standard value. Insuch a case, the voltage required to transfer the toner from theintermediary transfer belt 44 b to the recording material S may beinsufficient with the set secondary transfer voltage using the presetdefault recording material part voltage Vp. Therefore, in this case, itis desired to increase the set voltage of the secondary transfer voltageby increasing the recording material part voltage Vp. On the contrary,depending on the type and condition of the recording material S actuallyused for image formation, the water (moisture) content of the recordingmaterial S may have increased, with the result that the electricalresistance is lower than the value assumed as a standard value, andtherefore, the electrical discharge may be likely to occur. In thiscase, with the setting voltage of the secondary transfer voltage usingthe preset default recording material part voltage Vp, image defects mayoccur due to the abnormal discharge. Therefore, in this case, it isdesirable to lower the set voltage of the secondary transfer voltage byreducing the recording material part voltage Vp.

Therefore, it is desired that the operator such as a user or a serviceperson adjusts (changes) the recording material part voltage Vpdepending on the recording material S actually used for image formation,for example, to optimize the setting voltage of the secondary transfervoltage during the execution of the job (during the image formation). Inother words, it is only required that an optimum “recording materialpart voltage Vp+Vb (adjusting amount)” depending on the recordingmaterial S actually used for image formation is selected. Thisadjustment may be performed by the following method. For example, theoperator outputs the images while switching the secondary transfervoltage for each recording material S, and confirms the presence orabsence of an image defect occurring in the output image to obtain anoptimal secondary transfer voltage, on the basis of which settingvoltage (specifically, (recording material part voltage) Vp+(adjustingamount) ΔV) of the optimum secondary transfer voltage is determined.However, in this method, since the outputting operation of the image andthe adjustment of the setting voltage of the secondary transfer voltageare repeated, the recording material S which is wasted increases, and ittakes time in some instances.

In the case of this embodiment, the image forming apparatus 2 isoperable in the adjustment mode in which the secondary transfer voltageset during the image formation is adjusted. In this operation in theadjustment mode, an adjusting chart (test chart) on which a plurality ofrepresentative color patches (test toner images) are formed everyvoltage (for each test voltage) is outputted on the recording material Swhich is actually used for image formation, while the secondary transfervoltage is switched. And, the secondary transfer voltage (specifically,(recording material part voltage) Vp+(adjusting amount) ΔV) set duringthe image formation is adjusted on the basis of an acquired result(information on image density) of the reading of patches on theoutputted adjusting chart (recording material) by the image readingportion 80. In the case of this embodiment, on the basis of brightnessdata (density information) of a solid patch (solid image patch) on theadjusting chart, information on the adjusting amount ΔV for setting asecondary transfer voltage for optimizing a solid image density iscapable of being presented. By this, necessity that the operatorconfirms the presence or absence of the image defect by eye observationis reduced, so that it becomes possible to adjust the set voltage to aset voltage of a more appropriate secondary transfer voltage whilealleviating an operation load of the operator.

However, at the setting voltage of the secondary transfer voltageadjusted on the basis of the result of reading of the patch as describedabove, the secondary transfer voltage (absolute value) is excessivelyhigh and the “white void” occurs in some cases. This is because it isdifficult to discriminate occurrence or non-occurrence of the “whitevoid” by using an average image density of the patch acquired from thebrightness data of the patch. However, it has been known that therecording material part voltage at which the “white void” is liable tooccur has a correlation to the thickness of the recording material S.Further, the present inventors have found that the occurrence ornon-occurrence of the white void can be discriminated by a variation(specifically, a brightness dispersion value described later) in dividedregions of the patch (test toner image) through an experiment.

Therefore, in this embodiment, as described later, the occurrence ornon-occurrence of the white void is discriminated on the basis of thedensity of the patch formed on the adjusting chart. Then, an “adjustingamount ΔV” necessary to adjust the set voltage (recording material partvoltage Vp+(adjusting amount ΔV) of the secondary transfer voltage canbe made different between the case where the white void occurs and thecase where the white void does not occur.

<Adjusting Chart>

In this embodiment, in the operation in the adjustment mode, brightnessdata of the patch is acquired by reading an outputted adjusting chart bythe image reading portion 80, and a recommended adjusting amount of theset voltage of the secondary transfer voltage is presented. Further, inthis embodiment, the operator visually recognizes the outputtedadjusting chart in the operation in the adjustment mode, so that it isalso possible to change the adjusting amount presented as describedabove.

When confirmation of the outputted chart through eye observation by theoperator is also taken into consideration, the larger the patch size ofthe adjusting chart that is outputted in the operation in the adjustmentmode, the more advantageous it becomes since then it is easier to checkfor image defects. However, if the patch is large, the number of patcheswhich can be formed on one recording material S is reduced. The patchshape can be square and so on. The color of the patch can be determinedby the image defect to be checked and by the easiness of checking. Forexample, when the secondary transfer voltage is increased from a lowvalue, the lower limit of the secondary transfer voltage can bedetermined from the voltage value at which the secondary-color patchessuch as red, green, and blue can be properly transferred. In addition,in the case where the operator confirms the outputted chart by eyeobservation, when the secondary transfer voltage is further increased,the upper limit value of the secondary transfer voltage can bedetermined from the voltage value at which image failure (defect) occursdue to the high secondary transfer voltage in the halftone patch.

The adjusting chart usable with the operation in the adjustment mode inthis embodiment will be described. In the operation in the adjustmentmode in this embodiment, two types of image data (100A and 100B) shownin FIG. 6 and parts (a) and (b) of FIG. 7 are used for output of anadjusting chart 100. FIG. 6 shows image data of the adjusting chart(hereinafter also referred to as “large chart data”) 100A outputted tothe recording material S having a length in the process advancedirection of 420 to 487 mm. Parts (a) and (b) of FIG. 7 show image datafor a first side and a second side of the adjusting chart (hereinafteralso referred to as “small chart data”) 100B outputted to the recordingmaterial S having a length in the process advance direction of 210 to419 mm. In this embodiment, as the adjusting chart image data, only twotypes of image data shown in FIG. 6 and parts (a) and (b) of FIG. 7 areset. And, in the operation in the adjustment mode, the adjusting chartcorresponding to the image data cut out from any one of the two types ofimage data shown in FIG. 6 and parts (a) and (b) of FIG. 7 depending onthe size of the recording material S to be used is outputted on therecording material S. At this time, in this embodiment, image datahaving a size obtained by subtracting the margins at the end of therecording material S from the image data shown in FIG. 6 and parts (a)and (b) of FIG. 7 is cut out.

Here, in this embodiment, the maximum size (maximum sheet passing size)of the recording material S on which the image forming apparatus 2 canform an image is 13 inches×19.2 inches (short edge feeding). Inaddition, herein, the direction in which the recording material S is fedin the secondary transfer portion 45 n is referred to as the “processadvance direction” and the direction substantially perpendicular to theprocess advance direction is referred to as a “longitudinal direction”.

The large chart data 100A shown in FIG. 6 will be further described. Thelarge chart data 100A corresponds to the maximum sheet passing size ofthe image forming apparatus 2 of this embodiment, and the image size isapprox. (longitudinal direction) 13 inches (≈330 mm) at the shortside)×(process advance direction) 19.2 inches (≈487 mm) at the longside. When the size of the recording material S is 13 inches×19.2 inches(short edge feeding) or less and more than A3 size (short edge feeding),the adjusting chart corresponding to image data cut out from this largechart data 100A according to the size of the recording material S isoutputted. At this time, in this embodiment, the image data is cut outfrom the large chart data 100A in accordance with the size of therecording material S based on the leading end center with respect to theprocess advance direction. For example, in the case where of theadjusting chart 110 is outputted to the recording material S of A3 size(short edge feeding) (short side 297 mm×long side 420 mm), the imagedata having a size of 292 mm (short side)×415 mm (long side) is cut outfrom the large chart data 100A. And, the image corresponding to thecut-out image data is outputted on an A3 size recording material S witha margin of 2.5 mm at each end portion with the leading end center beingthe reference position with respect to the process advance direction.

The large chart data 100A includes one blue solid patch 101, one blacksolid patch 102, and two halftone patches 103 (gray (black halftone) inthis embodiment) arranged in the longitudinal direction. And, elevensets of patch sets (101 to 103) in the longitudinal direction arearranged in the process advance direction. The blue solid patch 101 andthe black solid patch 102 are each 25.7 mm×25.7 mm square (one side issubstantially parallel to the longitudinal direction). In addition, eachof the halftone patches 103 at both ends has a width of 25.7 mm in theprocess advance direction, and extends to the end of the large chartdata 100A in the longitudinal direction. In addition, the intervalbetween the patch sets 101 to 103 in the process advance direction is9.5 mm. The secondary transfer voltage is switched at the timing whenthe portion on the adjusting chart corresponding to this interval passesthrough the secondary transfer portion 45 n. The 11 patch sets (101-103)in the process advance direction of the large chart data 100A are withinthe range of 387 mm in the process advance direction such that when thesize of the recording material S is A3, they are within the length 415mm of the recording material S in the process advance direction. Inaddition, in this example, the large chart data 100A includesidentification information 104 for identifying the setting of thesecondary transfer voltage applied to each patch set in conjunction witheach of 11 patch sets (101 to 103) in the process advance direction. Inthis embodiment, this identification information 104 corresponds to anadjusted (adjustment) value described later. In this embodiment, elevenpieces of identification information 104 (−5 to 0 to +5 in thisembodiment) corresponding to eleven steps of secondary transfer voltagesettings are provided.

When the eye observation by the operator is also taken intoconsideration, the size of the patch is required to be large enough topermit the operator to easily determine whether there is an image defector not. For the transferability of blue solid patch 101 and black solidpatch 102, if the size of the patch is small, it can be difficult todiscriminate the defect, and therefore, the size of the patch ispreferably 10 mm square or more, and, more preferably, is 25 mm squareor more. The image defects due to abnormal discharge which occur whenthe secondary transfer voltage is increased in the halftone patch 103are often in the form of white spots. This image defect tends to be easyto discriminate even in a small size image, compared to thetransferability of the solid image. However, it is easier to observe ifthe image is not too small, and therefore, in this embodiment, the widthof the halftone patch 103 in the process advance direction is the sameas the width of the blue solid patch 101 and the black solid patch 102in the process advance direction. In addition, the interval between thepatch sets (101 to 103) in the process advance direction may be set sothat the secondary transfer voltage can be switched.

Here, it is preferable to prevent patches from being formed in theneighborhood of the leading and trailing ends of the recording materialS in the process advance direction (for example, in the range of about20 to 30 mm inward from the edge). This is because there may be an imagedefect that occurs only at the leading end or the trailing end, and isbecause, it may be difficult to determine whether or not an image defecthas occurred due to the secondary transfer voltage. Incidentally, thesolid image is an image with a maximum density level. In addition, inthis embodiment, the half-tone image corresponds to an image with atoner application amount of 10% to 80% when the toner application amountof the solid image is 100%.

Using the large chart data 100A described above, as the size of therecording material S becomes smaller than 13 inches (A3 size or more),the length, in the longitudinal direction, of the halftone patch 103 atboth ends in the longitudinal direction becomes smaller. In addition,using the large chart data 100A as described above, as the size of therecording material S becomes smaller than 13 inches (however, A3 size ormore), the margin at the trailing end in the process advance directionbecomes smaller.

The small chart data 100B shown in parts (a) and (b) of FIG. 7 will befurther described. The small chart data 100B corresponds to a sizesmaller than the A3 size, and the image size is approximately long side(longitudinal direction) 13 inches (≈330 mm)×short side (process advancedirection) 210 mm. If the size of the recording material S is A5 (shortside 148 mm×long side 210 mm) (longitudinal feed) or more and smallerthan A3 size (longitudinal feed), an adjusting chart corresponding tothe image data cut out of the small chart data 100B depending on thesize of the recording material S is outputted. In this embodiment, theimage data is cut out of the small chart data 100B in accordance withthe size of the recording material S on the basis of the leading endcenter with respect to the process advance direction. When the smallchart data 100B is used, two adjusting charts are outputted in order toincrease the number of patches.

The small chart data 100B has the same patches as those of the largechart data 100A. And, in the small chart data, five sets of patch sets(101 to 103) in the longitudinal direction are arranged in the processadvance direction. The five patch sets (101 to 103) in the processadvance direction of the small chart data 100B are arranged in a rangeof 167 mm in length in the process advance direction. In addition, inthis example, the small chart data 100B is provided with identificationinformation 104 for identifying the setting of the secondary transfervoltage applied to each set of patch sets, in association with therespective ones of the five patch sets (101 to 103) in the processadvance direction. And, on the first sheet, based on the small chartdata 100B shown in part (a) of FIG. 7 , five pieces of identificationinformation 104 (−4 to 0 in this embodiment) corresponding to thesetting of the lower secondary transfer voltage in five steps arearranged. In addition, on the second sheet, based on the small chartdata 100B shown in part (b) of FIG. 7 , five (+1 to +5 in thisembodiment) identification information 104 corresponding to higherfive-level secondary transfer voltage settings are arranged.

Using the above small chart data 100B, as the size of the recordingmaterial S becomes smaller (however, smaller than the A3 size and largerthan the A5 size), the length, in the longitudinal direction, of thehalftone patch 103 at both ends in the longitudinal direction becomessmaller. In addition, using the small chart data 100B as describedabove, as the size of the recording material S becomes smaller (however,smaller than the A3 size and larger than the A5 size), the margin at thetrailing end in the process advance direction becomes smaller.

Here, in this embodiment, not only a standard size but also an arbitrarysize (A5 size or more, 13 inches×19.2 inches or less) recording materialS is usable by an operator inputting and designating on the operatingportion 70 or the external device 200.

<Adjustment Mode>

Next, the operation in the adjusting mode in this embodiment will bedescribed. FIG. 8 is a flowchart showing the operation in the adjustmentmode in this embodiment. In addition, FIG. 9 is a schematic illustrationof a setting screen displayed on the operating portion 70. Thecontroller 30 executes the operation in the adjusting mode in the casewhere the operator provides an instruction to execute the adjustmentmode operation via the operating portion 70 of the image formingapparatus 2.

During the operation in the adjusting mode, the controller 30(adjustment process portion 31 d) causes the operating portion 70 todisplay a setting screen (not shown) for the kind and size of therecording material S and prompts the operator to input the kind and thesize of the recording material S on which the operator desires to formthe image (S1). The controller 30 (adjustment process portion 31 d) thenacquires information on the kind and size of the recording material Sdesignated by the operator through the operating portion 70.

Next, the controller 30 causes the operator to set the central voltagevalue of the secondary transfer voltage applied at the time of adjustingchart output, and whether to output the adjusting chart to one side orboth sides of the recording material S (S2). In this embodiment, inorder to be able to adjust the secondary transfer voltage duringsecondary transfer to the front side (first side) and back side (secondside) in double-side printing, the adjusting chart can be outputted onboth sides of the recording material S also in the operation in theadjustment mode.

In order to make the above-described setting, the controller 30 causesthe operating portion 70 to display an adjustment mode setting screen 90as shown in FIG. 9 . The setting screen 90 has a voltage setting portion91 for setting the center voltage value of the secondary transfervoltage for the front and back sides of the recording material S. Inaddition, the setting screen 90 has an output side selection portion 92for selecting whether to output the adjusting chart to one side or bothsides of the recording material S. Furthermore, the setting screen 90includes an output instruction portion (test page output button) 93 forinstructing adjusting chart output, a confirmation portion 94 (OK button94 a or the apply button 94 b) for confirming the setting, and a cancelbutton 95 for canceling the setting change.

When adjusting value “0” is selected in voltage setting portion 91, apreset voltage (more specifically, the recording material part voltageVp) set in advance for the currently selected recording material S isselected. And, the case that adjusting value “0” is selected will beconsidered in which 11 sets of patches from −5 to 0 to +5 when largechart data is used, and 10 sets of patches from −4 to 0 to +5 when smallchart data is used, are switched and applied as the secondary transfervoltages. In this embodiment, description will be made on assumptionthat the large chart data is used and the adjusting chart including the11 sets of patches is outputted. In this embodiment, the difference insecondary transfer voltage for one level is 150V. The controller 30acquires information relating to the setting such as the center voltagevalue set by way of the setting screen 90 in the operation portion 70.

Next, when the output instruction portion 93 on the setting screen 90 isselected by the operator, the controller 30 acquires information on theelectric resistance of the secondary transfer portion N when therecording material S is absent in the secondary transfer portion N (S3).In this embodiment, the controller 30 acquires a polynomial expression(quadratic expression in this embodiment) of two or more terms (terms ofthe second degree or more) with respect to a voltage-currentrelationship depending on the electric resistance of the secondarytransfer portion N by an operation similar to the operation in theabove-described ATVC. The controller 30 writes information on thisvoltage-current relationship in the RAM 33.

Then, the controller 30 causes the image forming apparatus to output thechart (S4). At this time, the controller 30 cuts out the chart data asdescribed above on the basis of the size information of the recordingmaterial S acquired in S1 and causes the image forming apparatus tooutput the adjusting chart on which the 11 sets of patches aretransferred while changing the secondary transfer voltage every 150 V.For example, it is assumed that the recording material part voltage inthe present environment is 900 V, and the secondary transfer portionpart voltage Vb acquired from the result of the ATVC is 1000 V. In thiscase, by applying from 1150 V to 2650 V, the adjusting chart, on whichthe 11 sets of patches are transferred while changing the secondarytransfer voltage in increments of 150 V, is formed. At this time, thecontroller 30 causes the current detection sensor 76 b to detect a valueof the current flowing during application of voltages of respectivevoltage levels, and acquires information on the electric resistances ofthe secondary transfer portion 45 n and the recording material S whenthe recording material S is present in the secondary transfer portion 45n (S5). In this embodiment, the controller 30 acquires, from a detectionresult of currents for voltages of 11 levels, the polynomial expression(quadratic expression in this embodiment) of two or more terms withrespect to the voltage-current relationship depending on the electricresistances of the secondary transfer portion N and the recordingmaterial S. The controller 30 writes the information on thevoltage-current relationship in the RAM 33. Incidentally, the currentwhen the recording material S is present in the secondary transferportion N may typically be detected during transfer of the patch, butmay also be detected at a portion of the recording material S wherethere is no toner before and after the patch for each voltage level.

Then, the controller 30 acquires the recording material part voltageVp(n) at each of the voltage levels from the relationship (quadraticexpression) between the voltage and the current, when the recordingmaterial S is present in the secondary transfer portion 45 n, acquiredin S5 and from the relationship (quadratic expression) between thevoltage and the current, when the recording material S is present in thesecondary transfer portion 45 n, acquired in S3 (S6). Here, “n”represents each of the voltage levels, and in this embodiment, “n”ranges from “1 to 11” corresponding to the 11 levels (11 sets ofpatches). Further, the voltage value of each voltage level isrepresented by Vtr(n). Further, the voltage value calculated by applyingeach level to the relationship (quadratic expression) between thevoltage and the current, when the recording material S is absent in thesecondary transfer portion N, acquired in S3 is represented by Vb(n). Atthis time, recording material part voltage Vp(n) at each voltage levelis represented by “Vp(n)=Vtr(n)−Vb(n)”.

Then, the outputted adjusting chart is supplied by the operator to theimage reading portion 80 by using the automatic original feeding device81, for example, so that the adjusting chart is read by the imagereading portion 80 (S7). At this time, the image reading portion 80 iscontrolled by the controller 30, and in this embodiment, RGB brightnessdata (8 bit) of each of the solid blue and black patches on theadjusting chart are acquired. Incidentally, when the adjusting chart isoutputted, the controller 30 is capable of causing the operating portion70 to display a message prompting the operator to supply the outputtedadjusting chart to the image reading portion 80.

Next, the controller 30 acquires an average of values of the brightnessof each of the solid blue and black patches in accordance with formula(1) appearing hereinafter by using the brightness data (density data)acquired in S7 (S8). By this process of S8, as an example, an average ofthe values of the brightness of the solid blue patches corresponding tothe respective voltage levels is shown in FIG. 10 . In FIG. 10 , theabscissa represents the adjusted (adjustment) values (−5 to 0 and 0 to+5) showing the respective voltage levels, and the ordinate representsthe average of the values of the brightness of the solid blue patches.Incidentally, in the case of this embodiment, as regards the solid bluepatches brightness data with B brightness was used, and as regards thesolid black patches, brightness data with G brightness was used. Anaverage brightness value B_(ave) (n) shown in the formula (1) is aparameter which reflects density. The formula (1) shows that at a loweraverage brightness value B_(ave) (n), the density of the toner imagetransferred on the recording material S is higher.

$\begin{matrix}\left( {{formul}(1)} \right) &  \\{{B_{ave}(n)} = {\frac{1}{M} \times {\sum\limits_{m = 1}^{M}{B(m)}}}} & (1)\end{matrix}$

Then, the controller 30 calculates a brightness dispersion value of thesolid black patches 102 in accordance with a formula (2) appearinghereinafter (S9). Here, an acquiring method of the brightness data usedfor calculating the brightness dispersion value of the solid blackpatches 102 will be described using FIG. 11 . As shown in FIG. 11 , areading region P is set in an image region of each of the solid blackpatches 102. In this embodiment, a size of the reading region P was 10mm×10 mm at a central portion of each patch. In order to calculate thebrightness dispersion value, the reading region P is divided into Mpieces of regions from P(1) to P(M), and brightness data in thecorresponding divided regions of the adjusting chart read in S7 arestored as B(1) to B(M), respectively, in the RAM 33. Each of sizes ofthe divided regions P(1) to P(M) may be minimum unit of readableresolution by the image reading portion 80 and may be about 300 dpi to1200 dpi, for example.

In the formula (2) for deriving the brightness dispersion value, “B(m)”represents brightness data of the divided region read “m-th” (m=1 to M)in the solid black patches, and M represents a total number of thedivided regions. “B_(ave)(n)” represents the average brightness value,and “D(n)” represents the brightness dispersion value of the solid blackpatches. The brightness dispersion value shown in the formula (2)reflects a transfer property in the case where the recording material Swas provided with unevenness. The formula (2) shows that at a largerbrightness dispersion value D(n) (dispersion), the transfer property ofthe toner image from the intermediary transfer belt 44 b onto therecording material S varies in larger degree depending on the region,i.e., that a variation in density is larger.

$\begin{matrix}\left( {{formul}(2)} \right) &  \\{{D(n)} = {\frac{1}{M} \times {\sum\limits_{m = 1}^{M}\left( {{B(m)} - {B_{ave}(n)}} \right)^{2}}}} & (2)\end{matrix}$

Next, the controller 30 acquires an adjusting value Na at which theaverage brightness value acquired in S8 becomes lowest (S10). In thecase of an example shown in FIG. 10 , the brightness becomes smallerwith an increasing adjusting value from “−5” toward “+2”. In thisadjusting value range, the electric field necessary to transfer thesolid blue patches becomes insufficient, and the transfer property ofthe solid blue patches is improved with the increasing adjusting value.The brightness value is minimum at further increased adjusting values“+3 to +4”. When the secondary transfer voltage is higher thannecessary, a risk of image defect resulting from an electric dischargephenomenon such as the white void increases, and therefore, in thisembodiment, a smaller adjusting value “+3” is selected as the adjustingvalue Na. Acquisition of the adjusting value Na as described abovecorresponds to selection of a first test voltage relatively large inaverage density.

Then, the controller 30 checks whether or not the “white void” occurs atthe adjusting value Na acquired in S10 (S11). In other words, thecontroller 30 checks whether or not the “white void” occurs in the solidblue patches formed at the first test voltage selected in S10.

A method of discriminating occurrence or non-occurrence of the “whitevoid” will be described using FIGS. 12 and 13 . A relationship betweenan occurrence status of the white void and the brightness dispersionvalue (calculated by the method of S9) in the case where the image isformed while changing the secondary transfer voltage by using the imageforming apparatus 2 is shown in FIG. 12 . Recording materials 1-6 (SHEET1 to SHEET 6) represent recording materials of different kinds (paperkinds) such as thin paper, plain paper, thick paper, and the like. Awhite void rank which is the abscissa represents a numerical value bywhich an occurrence status of the white void is represented by each offive ranks (levels) through eye observation. Specifically, the casewhere the white void does not occur is represented by a “rank 5”, andthe white void occurs in a larger amount with a lower numerical value ofthe ranks, so that the case where the white void occurs in the largestamount is represented by a “rank 1”.

As can be understood from FIG. 12 , the brightness dispersion value(absolute value) is different depending on the kind (paper kind) of therecording material. Further, as the white void more frequently occurs, avariation in density more frequently occurs, so that the brightnessdispersion value increases. In this embodiment, by utilizing thischaracteristic, an inclination of the brightness dispersion valuerelative to the adjusting value is acquired, and when the acquiredinclination of the brightness dispersion value is larger than aninclination of a predetermined rectilinear line L, discrimination of“white void occurrence” is made. The inclination of the rectilinear lineL was “1” on the basis of data.

Details of the discrimination of the “white void occurrence” will bedescribed using FIG. 13 . The inclination of a black brightnessdispersion value is calculated in the neighborhood of the adjustingvalue at which an average blue brightness value becomes minimum.Specifically, in this embodiment, an “adjusting value (Na) at which theaverage blue brightness value calculated in S10 becomes minimum”, an“adjusting value (Na−1) smaller than the adjusting value (Na) by 1”, andan “adjusting value (Na+1) larger than the adjusting value (Na) by 1”were used. The inclination of an approximate rectilinear line I of theblack brightness dispersion value in the above-described adjusting valuerange is calculated by the method of least square, and when theinclination of the approximate rectilinear line I is larger than therectilinear line L, the discrimination of the “white void occurrence” ismade. In other words, in the case where an increase in density variationof the black solid patches with an increase in secondary transfervoltage in the neighborhood of the first test voltage is a threshold ormore, the discrimination of the white void occurrence is made.

In the case of FIG. 13 , the inclination of the rectilinear line I is“2.25”, and therefore is larger than the inclination (1) of therectilinear line L, so that the discrimination of the “white voidoccurrence” is made. Incidentally, in this embodiment, thediscrimination is made by the inclination of the density distribution atthe adjusting value, and therefore, strictly, the case where a whitevoid rank is made worse by 1 per adjusting value of +1 is discriminatedas the “white void occurrence”.

In the case where the discrimination of the “white void occurrence” wasmade (Yes of S11), the controller 30 makes correction of the adjustingvalue (S12). FIG. 14 shows a relationship between a recording materialpart voltage of the secondary transfer voltage and the occurrence ornon-occurrence of the “white void” in the case where the image on therecording material S was checked with use of the image forming apparatus2 of this embodiment in an NL environment (temperature: 23° C.,humidity: 5%).

As shown in FIG. 14 , it turns out that as the thickness of therecording material S becomes thick, the recording material part voltage(absolute value) at which the “white void” occurs becomes larger.According to study by the present inventor, the recording material partvoltage at which the “white void” is liable to occur well coincides withan electric discharge start voltage acquired from the Paschen curve inthe case where the thickness of the recording material S is regarded asair (gap). That is, the relationship shown in FIG. 14 coincides withcause of occurrence of the “white void” such that the recording materialS is discharged during the secondary transfer and the toner at thedischarged portion is reversed in charge polarity and thus is nottransferred onto the recording material S. Therefore, in thisembodiment, by utilizing the above-described correlation, an upper limitof the recording material part voltage is provided depending on theinformation on the thickness of the recording material S. As a result,it becomes possible to select the adjusting value of the setting voltageof the secondary transfer voltage within a range in which the occurrenceof the “white void” can be suppressed.

Specifically, in this embodiment, the controller 30 extracts, from therecording material part voltage Vp(n) acquired in S6, a value which doesnot exceed the upper limit set depending on the information on thethickness of the recording material S. In this embodiment, every kind(paper category) of the recording material S in the market, such as“thin paper, plain paper, thick paper 1, thick paper 2, . . . ”, arelationship between the information (basis weight in this embodiment)on the thickness of the recording material S and the upper limit of therecording material part voltage Vp(n) is acquired in advance. Therelationship between the kind of the recording material S and therecording material part voltage Vp(n) is stored, as the table data asshown in FIG. 15 , in the ROM 32. The controller 30 makes reference tothe table data of FIG. 12 and acquires the upper limit of the recordingmaterial part voltage Vp(n) corresponding to the kind of the recordingmaterial S acquired in S1.

A conceptual image of the adjusting value correction in this embodimentin the case where the discrimination of the white void occurrence wasmade is shown in FIG. 16 . As shown in FIG. 16 , the adjusting value Nacalculated at the minimum blue brightness value is “+3”, but anadjusting value range in which the recording material part voltage Vp(n)is an upper limit value or less is “+2” or less. In this case, anadjusting value of “+2” which is smallest in adjusting value bluebrightness value in the adjusting value range of “+2” or less isemployed (as an adjusting value Na′). That is, in the case where theincrease in density variation of the solid black patches with theincrease in secondary transfer voltage in the neighborhood of the firsttest voltage is the threshold or more, a second test voltage lower thanthe first test voltage is employed as the transfer voltage set forduring the image formation. This second test voltage is a largest testvoltage of a plurality of test voltages in a range in which the secondtest voltage is lower than a predetermined voltage (an upper limit valueof Vb+Vp(n)) determined in advance depending on information on thethickness of the recording material S used for outputting the adjustingchart.

On the other hand, in the case where the discrimination ofnon-occurrence of the white void was made (S11 of FIG. 8 ), theadjusting value Na acquired in S10 of FIG. 8 is employed as it is. Thatis, in the case where the increase in the density variation of the solidblack patches with the increase in the secondary transfer voltage in theneighborhood of the first test voltage is the threshold or less, thefirst test voltage is employed as the transfer voltage for during theimage formation.

Next, the controller 30 causes the operating portion 70 to display the“adjusting value Na” or the “adjusting value Na” acquired as describedabove at the setting screen 90 (voltage setting portion 91) as shown inFIG. 9 (S13). The operator is capable of discriminating whether or notthe displayed adjusting value is appropriate, on the basis of thedisplay contents of the setting screen 90 and the outputted adjustingchart, and is capable of changing the adjusting value depending on theassociated case. The operator selects the confirmation portion 94 (OKbutton 94 a, application button 94 b) of the setting screen 90 as it isin the case where the displayed adjusting value is not changed. On theother hand, the operator inputs a desired value to the voltage settingportion 91 of the setting screen 90 in the case where the operatordesires that the adjusting value is changed from the displayed adjustingvalue, and then selects the finalizing portion 94 (OK button 94 a,application button 94 b).

The controller 30 discriminates whether or not an adjusting value changeinstruction is received (S14), and in the case where the adjusting valuechange instruction is received (Yes of S14), the controller 30 causesthe RAM 33 (or the secondary transfer voltage storage/operation portion31 f) to store the inputted adjusting value. In the case where theadjusting value is not changed and the confirmation portion 94 isselected (No of S15), the controller 30 causes the RAM 33 (or thesecondary transfer voltage storage/operation portion 31 f) to store theadjusting value determined in S9 (S16). The operation in the adjustingmode is thus ended.

During execution of a subsequent job, the controller 30 calculates theadjusting amount ΔV as “ΔV=(adjusting value)×150 V” depending on theadjusting value stored in the operation in the adjustment mode until theoperation in the adjustment mode is subsequently executed, and uses thecalculated value in calculation of the secondary transfer voltage Vtrfor during normal image formation.

In the case where the adjusting amount used in calculation of thesecondary transfer voltage Vtr is determined on the basis of only theaverage brightness value as in the conventional constitution, theaverage patch brightness value becomes minimum in some instances at avalue not less than the upper limit value of the recording material partvoltage, so that there was a liability that an adjusting amount in whichthere is a possibility of the occurrence of the white void exists. Onthe other hand, according to this embodiment, the adjusting amount inwhich there is the possibility of the occurrence of the “white void” isavoided, so that an appropriate adjusting amount can be determined.

Incidentally, the information on the upper limit value of the recordingmaterial part voltage Vp(n) used in S12 described above is not limitedto use in setting as table data (see FIG. 15 ) as in this embodiment.For example, a relational expression (not shown) showing a relationshipbetween the information on the thickness of the recording material S andthe recording material part voltage Vp(n) at which the “white void” isliable to occur is acquired in advance and can be stored in the ROM 32.In this case, the information on the thickness of the recording materialS is acquired and the upper limit value of the recording material partvoltage Vp(n) can be acquired from the above-described relationalexpression.

Further, the information on the thickness of the recording material S isnot limited to classification by the kind of the recording material S.For example, in the above-described S1, the operator is capable ofdirectly inputting a value relating to the thickness of the recordingmaterial S, such as the thickness or the basis weight. Further, in thestep corresponding to S1 (see FIG. 8 ), the value relating to thethickness of the recording material S, such as the thickness or thebasis weight may also be acquired by using a measuring means (not shown)for measuring the value relating to the thickness of the recordingmaterial S. As the measuring means, for example, a known thicknesssensor using ultrasonic wave may only be required to be provided on aside upstream of the secondary transfer portion N with respect to theprocess advance direction of the recording material S.

In this embodiment, as the patches for acquiring the average brightnessvalue and the brightness dispersion value, the solid blue patches andthe solid black patches were used but are not limited thereto. Forexample, instead of the solid blue patch, a solid patch of red or greenwhich is a secondary color can be used, and a solid patch of a singlecolor of yellow, magenta, cyan or black can be used. However, use of athick color such as black (single color) is advantageous fordiscriminating the occurrence or non-occurrence of the “white void”.

In this embodiment, the case where the operation by the operator isperformed through the operation portion 70 of the image formingapparatus 2 and thus the operation in the adjustment mode is executedwas described as an example, but the operation in the adjustment modemay also be executed by operation through external device 20 such as apersonal computer. In this case, it is possible to make setting similarto the above-described setting, through the setting screen displayed atthe display portion of the external device 200, by a driver program forthe image forming apparatus 2 installed in the external device 200.

In this embodiment, the information on the electric resistance of thesecondary transfer portion N from a start of the operation in theadjustment mode when the recording material S is absent in the secondarytransfer portion N was acquired. As a result, the information on theelectric resistance of the secondary transfer portion N in conformitywith a situation when the adjusting amount for setting of the secondarytransfer voltage is acquired can be acquired. However, if allowed fromthe viewpoint of accuracy or the like, as the information on theelectric resistance of the secondary transfer portion N, for example, aresult of ATVC at the time of a start of the last job in which theoperation in the adjustment mode is executed may also be used.

In this embodiment, in the operation in the adjustment mode, controlusing display of the adjusting value corresponding to the adjustingamount ΔV was carried out, but control more directly using the displayof the adjusting amount ΔV may also be carried out.

In this embodiment, when the voltage-current relationship is acquired,the value of the current flowing during supply of the predeterminedvoltage was detected, but a value of the voltage generating duringsupply of a predetermined current value may also be detected.

Further, the rank itself used for the white void discrimination is basedon eye observation evaluation by the present inventors, and may be basedon different criteria. Further, the method of discriminating the whitevoid with use of the brightness dispersion value is not limited to theabove-described method in this embodiment, but for example, it is alsopossible to employ a method of discriminating the white void by adifference in brightness distribution between adjusting values beforeand after the occurrence of the white void, not the inclination.Further, the adjusting values used for the discrimination were Na, Na−1,and Na+1, but values other than these (three) values may be used.

As described above, in this embodiment, the occurrence or non-occurrenceof the “white void” was discriminated on the basis of the density of thepatches formed on the adjusting chart, and the adjusting amount ΔV wascapable of being made different between the case where the “white void”occurred in the patch and the case where the “white void” did not occur.In the case where the “white void” occurs, compared with the case wherethe “white void” does not occur, the adjusting amount ΔV is suppressedto a lower level. By this, the secondary transfer voltage determined bythe “(secondary transfer voltage Vp)+(adjusting amount ΔV)” is adjusteddepending on the occurrence or non-occurrence of the “white void”.Accordingly, in the constitution in which the image forming apparatus 2is operable in the adjusting mode in which the secondary transfervoltage set for during the image formation is adjusted on the basis ofthe adjusting chart on which the patches are formed, the secondarytransfer voltage can be adjusted to a secondary transfer voltage atwhich suppression of the occurrence of the “white void” and suppressionof the occurrence of “image density lowering” can be compatiblyrealized.

Second Embodiment

Next, an image forming apparatus according to a second embodiment willbe described. In the image forming apparatus according to theabove-described first embodiment, in the case where the white voidoccurs in the adjusting chart, the adjusting value was lowered so as tobe adjusted to the secondary transfer voltage at which the white voidoccurrence risk is low. However, with the lowering in adjusting value,the secondary transfer voltage is liable to be set at a low value,whereby these was a liability that the density of the secondary-colorlowers.

In view of this, in the second embodiment, during operation in theadjusting mode, the operator is capable of selecting whether priority isgiven to ensuring the density of the secondary-color although there is aliability of the occurrence of the white void or to suppressing thewhite void although there is a liability of the occurrence of thelowering in density of the secondary-color.

The operation in the adjusting mode in the second embodiment will bedescribed using FIGS. 17 to 19 while making reference to FIGS. 1 and 2 .FIG. 17 is a flowchart showing the operation in the adjusting mode inthe second embodiment. FIG. 18 is a schematic illustration showing anexample of a setting screen of the operation in the adjusting mode.Incidentally, in the flowchart shown in FIG. 17 , processes identical tothe processes in the operation in the adjusting mode (see FIG. 8 ) inthe above-described first embodiment are represented by the samereference symbols, and in the following, processes different from theoperation in the adjusting mode in the first embodiment will beprincipally described.

As shown in FIG. 17 , first, designation of the recording material S andsettings of the center voltage and double-side printing are performedsimilarly as in the first embodiment (S1, S2). In this embodiment,advanced setting 96 is added to the setting screen of FIG. 18 , and whenthe operator selects the advanced setting 96, the controller 30(adjustment process portion 31 d) causes the operating portion 70 todisplay an advanced setting screen 90 b shown in FIG. 19 .

In the advanced setting screen 90 b of FIG. 19 , in priority imageselection 97, whether priority is given to output of a “high-densityimage” or a “halftone image” is capable of being inputted. The“high-density image” refers to an image, such as a so-called solid imageor an image of a secondary-color (red, blue, green), for which the toneris used in a large amount and for which a large transfer voltage isneeded. The “halftone image” refers to an image of which color isprincipally a pale color and of which toner application amount per unitarea of the recording material S is relatively small. The white void isliable to be visualized in the case of the halftone image, andtherefore, in the case where the “halftone” is selected in the priorityimage selection 97, correction for during the occurrence of the whitevoid is made.

The operator selects a priority image and then selects a confirmationportion 98 of the advanced setting screen 90 b.

The controller 30 causes the RAM 33 or the like to store the settinginputted by the operator (S100). After the advanced setting iscompleted, the sequence returns to the setting screen 90 a of FIG. 18 ,and the controller 30 carries out the processes (S3 to S10) to thecalculation of the minimum brightness adjusting value (S10) similar tothe processes in the first embodiment.

Then, in the case of the second embodiment, before occurrence ornon-occurrence of the white void is discriminated (S11), the controller30 discriminates whether the priority image is the “high-density image”or the “halftone” image (S101). In the case where the priority image isthe “halftone” image (Yes of S101), the controller 30 makesdiscrimination of the occurrence or non-occurrence of the white voidsimilarly as in the case of the first embodiment (S11). On the otherhand, in the case where the priority image is the “high-density image”(No of S101), the controller 30 does not make the discrimination of theoccurrence or non-occurrence of the white void, and the sequence jumpsto S13. By such a discrimination flow, depending on the priority imageinputted by the operator, an appropriate secondary transfer voltage canbe set. Thereafter, a flow similar to the flow in the first embodimentis performed, and then the operation in the adjusting mode is ended.

Thus, the secondary transfer voltage can be easily adjusted to anoptimum secondary transfer voltage corresponding to a desired use statusby causing the operator to select whether priority is given to thesuppression of the lowering in secondary-color density or to thesuppression of the occurrence of the white void, and thus is preferred.

Incidentally, in this embodiment, the selection of the priority image ismade on the setting screen 90 a (S100), but the present invention is notlimited thereto. For example, the operator may select the priority imageon the basis of image information (image information from the imagereading portion 80 or the external device 200 (see FIG. 2 )) of theimage to be actually intended to be outputted by the operator. Then, inthe case where halftone image information is contained in a large amountin the image information, the process for during the white voidoccurrence (see FIG. 12 ) may be performed.

Incidentally, in the above-described embodiments, the image formingapparatus 2 of the intermediary transfer type in which the toner imagesare primary-transferred from the photosensitive drums 51 y to 51 k forthe respective colors onto the intermediary transfer belt 44 b andthereafter are secondary-transferred from the intermediary transfer belt44 b onto the recording material S was described as an example, but thepresent invention is not limited thereto. The above-describedembodiments are also applicable to an image forming apparatus of adirect transfer type in which the toner images are directly transferred,onto the recording material S, from the photosensitive drums 51 y to 51k, for the respective colors, which rotate while carrying the tonerimages.

According to the present invention, in the case of the constitution inwhich the operation in the adjusting mode in which the transfer voltageset for during the image formation is adjusted on the basis of the testchart on which the plurality of the test images are transferred, thetransfer voltage is capable of being adjusted to the transfer voltage atwhich the suppression of the occurrence of the “white void” and thesuppression of the occurrence of the “lowering in image density” arecompatibly realized.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2021-059836 filed on Mar. 31, 2021, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus comprising: an imagebearing member configured to bear a toner image; an image formingportion configured to form the toner image on said image bearing member;a transfer member configured to transfer the toner image from said imagebearing member onto a recording material; a power source configured toapply a transfer voltage, to said transfer member, for transferring thetoner image from said image bearing member onto the recording material;an acquiring portion configured to acquire information on an imagedensity of an image transferred on the recording material; and acontroller capable of executing an operation in an adjustment mode inwhich a test chart is outputted by transferring a plurality of testtoner images onto the recording material under application of aplurality of different test voltages from said power source and then onthe basis of densities of said test toner images transferred on the testchart, the transfer voltage set for transfer of the toner image fromsaid image bearing member onto the recording material is adjusted,wherein said test toner images include a first test toner imagetransferred onto the recording material under application of a firsttest voltage and a second test toner image transferred onto therecording material under application of a second test voltage differentfrom the first test voltage, and wherein in the operation in theadjustment mode, said controller adjusts the transfer voltage set forthe transfer on the basis of first information on variation in imagedensity acquired in different regions of said first test toner image andsecond information on variation in image density acquired in differentregions of said second test toner image.
 2. An image forming apparatusaccording to claim 1, wherein said controller acquires the informationon the variation on the basis of an average of the information on imagedensities acquired in the different regions of said test toner image andthe image densities acquired in the different regions.
 3. An imageforming apparatus according to claim 1, wherein the information on thevariation is a brightness dispersion value of said test toner image. 4.An image forming apparatus according to claim 1, wherein said controlleracquires an inclination of a density variation of said test toner imagerelative to said test voltage on the basis of the first information andthe second information, and adjusts the transfer voltage set for thetransfer on the basis of the inclination.
 5. An image forming apparatusaccording to claim 4, wherein said controller determines the first testvoltage as the transfer voltage set for the transfer when theinclination is less than a predetermined value, and determines thesecond test voltage, lower than the first test voltage, as the transfervoltage set for the transfer when the inclination is not less than thepredetermined value.
 6. An image forming apparatus according to claim 5,wherein the second test voltage is highest in said test voltages withina range lower than a predetermined voltage determined in advancedepending on information on a thickness of the recording material usedfor output of the test chart.
 7. An image forming apparatus according toclaim 1, wherein in the operation in the adjusting mode, said controlleroutputs the test chart by forming a plurality of secondary-color testtoner images and a plurality of single-color test toner images withsingle-color toner under application of said test voltages.
 8. An imageforming apparatus according to claim 1, further comprising an inputportion capable of inputting an instruction such that priority is givento output of a halftone image as an image formed during image formation,wherein when said controller receives input of the instruction, in theoperation in the adjusting mode, said controller adjusts the transfervoltage set for the transfer on the basis of the first information andthe second information.